I actually believe that most of what tomazzi said is not exactly true. For example, even the Debian motto is "the universal operating system" not "the server operating system". So to me appears that the Debian project have attempted to create a usable general purpose operating system for a wide range of processor architectures. In reality, of course, Debian is mostly used in server contexts.

tomazzi also said that Raspbian is almost entirely Debian and that Debian arm is abandoned. Putting aside the obvious contradiction in that statement itself, the Raspberry pi foundation have made significant modification and additions to almost every component of the OS, kernel, firmware, user space software (such as sonic pi and what more, also got many third parties to add sofware to the repos that are not in Debian like mathematica), they tweaked their own DE (based on LXDE but still), libraries for GPIO and the list goes on. Not only that, but ARM is actually one of the best maintained architectures on Debian.

Thirdly, it appears to me (from comments on linux questions) that GNOME touchscreen support actually works quite well now.

I think the number one reason why getting tweakable open source operating systems on smart phones is so hard is because the mainline kernel does not support most of the hardware. The manufacturers deliberately change the hardware at least once in a year and don't want their code open. If even one phone model/mark were to come out that would stay around for more than 18 months, there would be even the smallest amount of incentive to develop an OS image that is more permanent. Right now every OS image needs to be custom designed for that one phone model. Maybe something like Fairphone, where users might actully have access to spare parts etc. and therefore have motive to use the same phone for more than a year. Ubuntu Touch is probably the best competitor for Android as it allows updates to all ports forever, unlike Android, but as stated it relies heavily on Android and is not exactly FOSS.

btw. this same forum has a HOWTO to using Debian on an android phone already from 2013 viewtopic.php?f=16&t=106246 But there have been many other projects and kickstarters where people have created linux tablets and phones.